Noninvasive characterization of a flowing multiphase fluid using ultrasonic interferometry

ABSTRACT

An apparatus for noninvasively monitoring the flow and/or the composition of a flowing liquid using ultrasound is described. The position of the resonance peaks for a fluid excited by a swept-frequency ultrasonic signal have been found to change frequency both in response to a change in composition and in response to a change in the flow velocity thereof. Additionally, the distance between successive resonance peaks does not change as a function of flow, but rather in response to a change in composition. Thus, a measurement of both parameters (resonance position and resonance spacing), once calibrated, permits the simultaneous determination of flow rate and composition using the apparatus and method of the present invention.

STATEMENT REGARDING FEDERAL RIGHTS

[0001] This invention was made with government support under ContractNo. W-7405-ENG-36 awarded by the U.S. Department of Energy to TheRegents of The University of California. The government has certainrights in the invention.

FIELD OF THE INVENTION

[0002] The present invention relates generally to swept frequencyacoustic interferometric (SFAI) determination of sound velocity andabsorption in fluids and, more particularly, to the use of SFAI tononinvasively determine flow velocity and composition for flowingfluids.

BACKGROUND OF THE INVENTION

[0003] Swept frequency acoustic interferometry (SFAI) [1] is anadaptation of the techniques of ultrasonic interferometry developedseveral decades ago for determining sound velocity and absorption inliquids and gases. In the original technique, and also in more recentmodifications of the technique [2], the transducers (sensors) wereplaced in direct contact with the fluid being tested. This restrictedthe use of this technique to highly specialized laboratorycharacterization of fluids. By contrast, the SFAI technique extends thecapabilities of the ultrasonic interferometry technique significantlyand allows the noninvasive determination of velocity and attenuation ofsound in a fluid (liquid, gas, mixtures, emulsions, etc.,) inside sealedcontainers (pipes, tanks, chemical reactors, etc.) over a wide frequencyrange. In addition, if the container material properties (density andsound velocity) are known, the liquid density can be determined usingthe SFAI technique. It has also been shown that it is possible touniquely identify various chemical compounds and their most significantprecursors based on the physical parameters of sound: velocity,attenuation, frequency dependence of sound attenuation, and density [3].

[0004] Oil companies have recently shown interest in noninvasivetechniques for characterizing oil flow in pipes from oil fields.

[0005] U.S. Pat. No. 5,606,130 [4] states that it is anticipated thatthe SFAI measurements described therein can be performed on flowingsamples in pipes. However, no mention is made therein of how to performsuch measurements.

[0006] Accordingly, it is an object of the present invention to providean apparatus and method for determining the composition of flowingfluids.

[0007] Another object of the invention is to provide an apparatus andmethod for determining the flow rate of a fluid.

[0008] Additional objects, advantages and novel features of theinvention will be set forth in part in the description which follows,and in part will become apparent to those skilled in the art uponexamination of the following or may be learned by practice of theinvention. The objects and advantages of the invention may be realizedand attained by means of the instrumentalities and combinationsparticularly pointed out in the appended claims.

SUMMARY OF THE INVENTION

[0009] To achieve the foregoing and other objects, and in accordancewith the purposes of the present invention, as embodied and broadlydescribed herein, the method for monitoring the composition of a fluidflowing through a vessel hereof includes the steps of: applying acontinuous periodic acoustical signal to the outside of the vessel suchthat the acoustical signal is transferred to the flowing fluid, therebygenerating vibrational resonance features having a plurality of maximaand minima therein; detecting the vibrational features generated in theflowing liquid; sweeping the continuous periodic acoustical signalthrough a chosen frequency range which includes two chosen consecutivemaxima among the vibrational resonance features; and measuring thefrequency difference between the two chosen consecutive maxima of theflowing fluid, whereby changes in the composition of the fluid areidentified.

[0010] In another aspect of the present invention, in accordance withits objects and purposes, the method for monitoring the flow rate of afluid through a vessel hereof includes the steps of: applying acontinuous periodic acoustical signal to the outside of the vessel suchthat the acoustical signal is transferred to the flowing fluid, therebygenerating vibrational resonance features having a plurality of maximaand minima therein; detecting the vibrational resonance featuresgenerated in the flowing liquid; sweeping the continuous periodic signalthrough a chosen frequency range which includes two chosen consecutivemaxima in the standing-wave vibrational pattern; recording the frequencydifference between the two chosen consecutive maxima to determinewhether the composition of the fluid has changed; correcting thelocation of the resonance peaks in response thereto; and determining thefrequency of one chosen resonance peak, whereby the flow rate of thefluid is determined.

[0011] In yet another aspect of the present invention, in accordancewith its objects and purposes, the method for monitoring the compositionof a fluid flowing at a flow rate through a vessel hereof includes thesteps of: applying a continuous periodic acoustical signal to theoutside of the vessel such that the acoustical signal is transferred tothe flowing fluid, thereby generating vibrational resonance featureshaving a plurality of maxima and minima therein; detecting thevibrational features generated in the flowing liquid; sweeping thecontinuous periodic acoustical signal through a chosen frequency rangewhich includes one maximum among the vibrational resonance features;measuring the flow rate of the fluid; measuring the frequency of themaximum of the flowing fluid; and correcting the frequency of themaximum for the flow rate of the fluid, whereby changes in thecomposition of the fluid are identified.

[0012] In still another aspect of the present invention, in accordancewith its objects and purposes, the method for monitoring the flow rateof a fluid having a composition and flowing through a vessel hereofincludes the steps of: applying a continuous periodic acoustical signalto the outside of the vessel such that the acoustical signal istransferred to the flowing fluid, thereby generating vibrationalresonance features having a plurality of maxima and minima therein;detecting the vibrational features generated in the flowing liquid;sweeping the continuous periodic acoustical signal through a chosenfrequency range which includes one maximum among the vibrationalresonance features; measuring the frequency of the maximum of theflowing fluid; determining the composition of the fluid; and correctingthe frequency of the maximum for the composition of the fluid, wherebythe flow rate of the fluid is determined.

[0013] Benefits and advantages of the present invention include thenoninvasive measurement of flow rate and changes in composition of aflowing fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings, which are incorporated in and form apart of the specification, illustrate an embodiment of the presentinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings:

[0015]FIG. 1a is a schematic representation of one embodiment of theapparatus of the present invention showing a dual-element transducerlocated on one side of the pipe or tube through which the liquid flows,FIG. 1b shows a second embodiment of the apparatus of the presentinvention showing the transmitting transducer on one side of the pipe ortube and the receiving transducer on the other side thereof, and FIG. 1cshows a third embodiment of the present invention, wherein a singlepiezoelectric transducer is used for both generating an oscillatorysignal in the sample and for responding to the resonances producedthereby.

[0016]FIG. 2 shows an example of an electronic circuit suitable forobserving the resonance response of the fluid flowing through the tubeor pipe as a function of frequency; a similar apparatus would besuitable for observing changes in the phase of the fluid from that ofthe initial ultrasound signal impressed upon the tube or pipe by thetransmitting transducer as a function of changes in fluid composition orflow rate.

[0017]FIG. 3 is a composite resonance spectrum for a noninvasivemeasurement using a swept frequency apparatus and method of the presentinvention, and illustrates that liquid peaks can be studiedindependently of the resonances induced in the wall of the container ifan appropriate frequency region is selected.

[0018]FIG. 4 is a graph of the physical properties of several liquidsmeasured in a static container.

[0019]FIG. 5 shows swept frequency acoustic interferometry measurementsmade under flowing conditions, showing that the sound speed which isrelated to the spacing between the peaks for consecutive resonance doesnot change as a result of the flow, nor does the sound attenuation whichis related to the width of the resonance peaks.

[0020]FIG. 6 shows swept frequency acoustic interferometry measurementsmade in a liquid which contains bubbles; again, the spacing between thepeaks does not change.

[0021]FIG. 7 is a graph of the measured differential phase magnitude asa function of mass flow for water.

[0022]FIG. 8 shows the resonance patterns for water and oil as afunction of frequency and illustrates that at an appropriate frequencythe resonance peak characteristics are sensitive to the acousticproperties of the liquid.

DETAILED DESCRIPTION

[0023] Briefly, the present invention includes apparatus and method fornoninvasively monitoring both the flow and/or the composition of aflowing fluid using ultrasound. In what follows, fluid will be definedas a liquid, including liquids with more than one constituent, liquidswith some particulates and those containing gas bubbles. As will bedescribed in detail hereinbelow, it was found that the position of theresonance peaks for a fluid excited by a swept-frequency ultrasonicsignal change frequency both in response to a change in composition andin response to a change in the flow velocity thereof. Additionally, thefrequency difference between successive resonance peaks does not changeas a function of flow, but rather in response to a change incomposition. Thus, a measurement of both parameters (resonance positionand resonance spacing), once calibrated, permits the simultaneousdetermination of flow rate and composition using the apparatus andmethod of the present invention. Additional parameters useful fordetermining the fluid composition include the full-width-at-half-maximumof a resonance feature, the amplitude ratio and the acoustic impedanceof the liquid. None of these parameters was found to changesignificantly as a function of flow rate. The apparatus was tested usingdecane, dodecane, water, and brine solutions to determine whether thesecompositions readily distinguishable using the swept frequency acousticinterferometry (SFAI) technique that has been described in detail forstatic fluids in U.S. Pat. No. 5,767,407 [1] and U.S. Pat. No. 5,886,262[5], the teachings of both references being hereby incorporated byreference herein.

[0024] Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. Similar or identical structure are labeled usingidentical callouts. Turning now to FIG. 1a, a schematic representationof one embodiment of the apparatus of the present invention is shownillustrating a dual-element transducer or two, single-elementtransducers 10 a and 10 b, located on one side of the pipe or tube, 12,through which fluid, 14, flows, and electronics, 16, provide the fixedor variable acoustic driving frequency, 18, and receive the resonancesignal, 20, generated in fluid 14. FIG. 1b shows a second embodiment ofthe apparatus of the present invention showing transmitting transducer10 a powered by swept sine-wave generator, 20, on one side of pipe ortube 12 and receiving transducer 10 b in electrical connection withreceiving and analyzing electronics, 22, on the other side thereof.Examples of the circuitry and principles of operation are found in thedescription for the '232 patent, supra. For single-frequency excitationof resonances within the fluid 14, the change in phase can be monitoredby the apparatus. As will be demonstrated hereinbelow, tube or pipe 12can be fabricated from metals, plastics or glass. FIG. 1c shows a thirdembodiment of the present invention, wherein a single piezoelectrictransducer, 24, is used for both generating an oscillatory signal in thesample and for responding to the resonances produced thereby. As is alsodescribed in the description for the '262 patent, supra, bridge circuit,26, is employed to derive a differential signal and includes one armwhich contains transducer 24, a balancing arm which contains a matchingor equivalent circuit for the transducer, and a swept sine-wavegenerator. When the transducer is not attached to the pipe, the outputis zero; however, when attached to the pipe, a changing pipe impedancedue to standing waves generated therein generates a signal of one armrelative to that of the other arm and the output is the differencebetween these values.

[0025] For measurement of the flow rate, it is necessary to correct forchanges in the composition of the fluid, or at least have the knowledgethat the composition is not changing. There are numerous commerciallyavailable composition monitoring devices including real-time, on-linedevices such as infrared spectrometers, and uv/vis spectrometers, asexamples, and sampling devices such as liquid chromatographs and massspectrometers as examples. One might take a sample for analysis using asyringe introduced through a septum for off-site analysis. Anotherprocedure would be to stop the flow and utilize the SFAI proceduredetailed in Reference 1. Due to the number and variety of these methods,FIGS. 1a-1 c do not show any devices for monitoring the composition ofthe flowing fluid; except for those taught by the present claimedinvention. Similarly, for monitoring the composition of the fluid;certain embodiments of the present invention require that a correctionto the resonance peak location for the fluid flow rate be made, or atleast knowledge that the flow rate is constant. There are numerous andvaried commercially available flow measuring devices, some disposed inthe interior of a pipe through which the fluid is flowing, and othersdisposed on the exterior of the pipe. Again, no flow measuring devicesare illustrated in FIGS. 1a-1 c except for those taught by the presentclaimed invention, for the same reason as the fluid compositionmonitoring apparatus is not displayed.

[0026]FIG. 2 shows an example of an electronic circuit suitable forobserving the resonance response of the fluid flowing 14 through thetube or pipe 12 as a function of frequency; a similar apparatus would besuitable for observing changes in the phase of the fluid from that ofthe initial ultrasound signal impressed upon the tube or pipe by thetransmitting transducer as a function of changes in composition of thefluid and/or changes in fluid flow rate. The electronic circuitcomprises a direct digital synthesizer (DDS) IC, 28, for generatingfrequencies up to 10 MHz; amplifier, 30, for amplifying the outputsignal of transducer 10 b; phase detector, 32, for providing a voltageoutput proportional to the difference in phase between two sine-waves;analog-to-digital (A/D) converter, 34, having a minimum of two-channelmultiplexing capability, 36 MUX (multiplexer); microcontroller, 38having floating point calculation and fast Fourier transform (FFT)capability; and display unit, 40, for displaying the results. The twotransducers utilized were commercial, off-the-shelf piezoelectrictransducers (Panametric Videoscan 5 MHz center frequency, 0.5 in.diameter transducers). The actual brand is not critical to themeasurements and almost any transducer can be employed.

[0027] Microcontroller 38 is software programmable and controls DDS 28to generate sine-waves having a chosen frequency within the frequencyrange of the device. The frequency output of DDS 28 can either be fixedor varied with time (that is, swept). The frequency resolution of theapparatus used to demonstrate the present invention was better than 0.1Hz. The frequency could be swept over a chosen frequency range in afraction of a second.

[0028] The output of the DDS is used to excite the transmittertransducer 10 a placed in physical contact with pipe 10 through whichliquid 14 can be flowing. Second transducer 10 b is used as thereceiver. It is also possible to use a single transducer and measure theimpedance change thereof (FIG. 1c hereof) to make the same type ofmeasurement. However, for the present description, it is simpler todiscuss the two separated transducer embodiment which are placed inphysical contact with pipe 12 either on the same side thereof in thevicinity of one another or on opposite sides of pipe 12. Receivertransducer 10 b receives the signal response of the fluid/pipe to theexcitation signal from transducer 10 a which is amplified by amplifier30 with a gain of up to 60 dB. The amplified signal is processed usingmultiplexing input 36 of A-D converter 34. Microcontroller 38 controlsthe switching of multiplexer 36 input and the data output from A-Dconverter 34.

[0029] For phase measurements, phase detector 32 circuit is employedhaving as its output the phase difference between the signal totransmitter transducer 10 a and the amplified signal of receivertransducer 10 b. Typically, phase measurements are made at a fixedfrequency that corresponds to a resonance peak when there is no liquidflow through the pipe. When the liquid is allowed to flow, the phasedetector output is related to the magnitude of the flow. There is nosimple relationship to describe the phase difference as a function offlow and a calibration is required. The observed phase difference is anapproximately linear function of the flow (see FIG. 7 hereof.Microcontroller 38 can continuously monitor the phase output and convertthis to a flow value and display the results using display 40.

[0030] For fluid composition monitoring, the circuit switches to thechannel that directs the amplified receiver transducer signal output toA/D converter 34. For this measurement, the frequency applied to thetransmitter transducer is rapidly swept through a chosen frequencyrange. This range depends on the dimensions of the pipe (see FIG. 3hereof). Although any convenient frequency range may be employed, it ispreferred that a frequency range between two successive wall resonances(see FIG. 3 hereof) be used. This produces a flat baseline and theresults can be fitted to a theory involving simple equations. A briefdescription of the relationships follows.

[0031] As stated hereinabove, in order to readily obtain the acousticalproperties of a fluid, it is convenient to select a measurementfrequency range to avoid resonance contributions from the walls(approximately 4, 6, and 8 MHz in FIG. 3 as examples). To first order,this reduces the analysis essentially to that of sound transmissionthrough a one-layer model making the calculations more straightforwardwithout introducing substantial errors in the measurement of sound speedand sound attenuation. This is similar to avoiding the transducercrystal resonance frequency region in traditional interferometry. Theintensity transmission coefficient, T, for the case of a single fluidlayer having path-length, L, attenuation coefficient, α_(L) (α_(L)L<<1),and sound speed, c_(L), between two identical wall boundaries can beexpressed as $\begin{matrix}{{T = \frac{1}{\left( {1 + {\frac{1}{2}\sigma \quad \alpha_{L}L}} \right)^{2} + {\frac{\sigma^{2} - 4}{4}{\sin^{2}\left( {\frac{\omega}{c_{L}}L} \right)}}}},} & (1)\end{matrix}$

[0032] where, σ=z_(w)/z_(L)+z_(L)/z_(w), ω=2πf, is the angularfrequency, and z_(w) and z_(L) are the acoustic impedance of the walland fluid, respectively. For most liquids inside a metal container,σ≈z_(w)/z_(L). T in Eq. (1) is a periodic function of ωL/c_(L) andreaches a maximum (peak) value whenever the condition 2πf_(n)L/c_(L)=nπis satisfied, where f_(n) is the frequency of the n-th peak. From thiscondition, the sound speed c_(L) (c_(L)=2 L Δf) can be determined if thefrequency difference between successive peaks is measured.

[0033] As stated, the sound speed in the fluid is determined from thefrequency spacing between any two consecutive peaks. Therefore, oneneeds to sweep the frequency over a range that encompasses any twosuccessive resonance peaks. The digitized data of two resonance peakscan then be used to extract the sound speed since the liquid path length(the diameter of the pipe) is known. This is the most expedient mannerfor determining the sound speed in the fluid, and the measurement can bemade in a fraction of a second. If either greater accuracy or resolutionis required, a second approach may be used. In this approach, a muchlarger frequency range is covered such that multiple resonance peaks(say, 10) are observed. The microcontroller is used to perform a FFT ofthe data which determines the periodicity of the resonance peaks whichis directly related to the peak spacing. This is equivalent to averagingthe sound speed measurement over multiple peak spacings.

[0034] Sound attenuation and liquid density are related to the frequencyspectrum. The ratio of transmission coefficient minima, T_(min), andmaxima, T_(max), can be expressed in terms of σ and α_(L) as:$\begin{matrix}{\frac{T_{\min}}{T_{\max}} = {\frac{2}{\sigma} + {L\quad {{\alpha_{L}\left( f^{2} \right)}.}}}} & (2)\end{matrix}$

[0035] Equation (2) illustrates that both α_(L) and σ can be determinedfrom a linear fit of the data of the transmission ratio factor as afunction of f². The intercept at zero frequency is related to theacoustic impedance ratio σ. If the impedance of the wall material isknown, the liquid density can be determined since the sound speed of thefluid is independently determined as discussed hereinabove.

[0036] Another for determining the sound attenuation coefficient is toutilize the half-power bandwidth of observed resonance peaks. From Eq.(1), an inverse solution for the half-power bandwidth, δf, can bederived in terms of acoustic properties of the fluid according to$\begin{matrix}{{\delta \quad f} = {\frac{2\quad c_{L}}{\pi \quad \sigma \quad L} + {\frac{c_{L}{\alpha_{L}\left( f^{2} \right)}}{\pi}.}}} & (3)\end{matrix}$

[0037] Similar to Eq. (2), the second term is the contribution fromliquid sound absorption and is identical to the solution obtained fromresonator theory of transducers in direct contact with the liquid. Thefirst term, the width extrapolated to zero frequency δf₀, is independentof frequency and depends on σ, c_(L), and L. This term results from thereflection loss at the wall-liquid interface due to acoustic impedancemismatch and can be used to determine liquid density if the acousticimpedance of the wall is known. This analysis can be used to extract theabsolute value of the sound absorption of the liquid. More often,monitoring the peak width for the resonance peaks for say oil and water(see FIG. 8 hereof) to obtain qualitative discrimination is sufficient.The resonance width is the full-width-at-half-maximum of the peak, andthe microcontroller can rapidly calculate this quantity by fitting thetop part of any peak with a Lorentzian line shape. The Lorentzian can belinearized by inverting (taking the reciprocal of the amplitude at eachfrequency) the data and then a simple parabolic fit is all that isnecessary instead of a nonlinear least-squares fitting. The widthindicated for crude oil in FIG. 8 is meant only for qualitativedescription.

[0038] Thus, the spectrum contains all the information related to theliquid, any desired parameter can be extracted through simplecalculations.

[0039] The above description covers the behavior of sound transmissionthrough a fluid path as a function of frequency and Eq. (1) describedthe frequency spectrum. It is possible to least-squares curve-fit thisequation to observed experimental data to extract various parameters ofthe fluid, such as sound speed, sound absorption, and density. However,it is also possible to derive the same information with good accuracyeven by monitoring a single resonance peak. In practice, one oftenrequires to monitor the change in the quality of the fluid, in terms ofsound speed, sound absorption and density variation, flowing through apipe and not absolute values of these quantities. In such a situation,an electronic circuit simply selects and tracks a single resonance peakand measures the peak width, peak position, and the minimum value (thebaseline) of the resonance curve.

[0040] If the sound speed of the fluid changes, the selected resonancepeak position will change in frequency. This frequency shift (Δf_(s)) isrelated to the sound speed variation (Δc) simply as Δf_(s)=(n/2L) Δc.Here n, is the order number of the particular resonance is peak.Similarly, if the sound absorption of the liquid changes then theobserved difference in full-width at half-maximum (δf) value of theresonance peak for a selected resonance peak Δ(δf) provides the changein sound absorption as Δ (δf)≈(Δc/π)Δα. Another method for determiningthe change in sound absorption is to measure the ratio of the resonancepeak minimum, T_(min) to the peak maximum, T_(max) for a singleresonance feature. The change in absorption Δα=Δ(T_(min)/T_(max))/L (seeEq. (2) and FIG. 8 hereof. This approach provides a more rapid procedurefor determining Δα and does not require curve fitting.

[0041] Finally, the variation in the minimum (T_(min)) of the resonancecurve can provide a measure of the variation in the change in acousticimpedance of the liquid. The relationship between the two parameters canbe expressed as ΔT_(min)=(2/Z_(w)) ΔZ, where ΔZ is the change inacoustic impedance of the liquid. All these relationships are derivedfrom Eq. (1), and are shown graphically in FIG. 8 hereof for descriptionpurposes. For qualitative monitoring of variation in sound absorption,for example, for simple discrimination between oil and water, it isexpedient to simply determine the width of the resonance peak betweenthe maximum and minimum of the resonance as shown in FIG. 8. Foraccurate determination of sound absorption, it is better to fit theresonance spectrum with several peaks using Eq. (1).

[0042] By employing a phase-locked-loop circuit that simultaneouslymonitors both the resonance peak position of a single peak and the peakwidth in the most sensitive frequency region, both sound speed and soundattenuation are provided continuously. These values are then used tocharacterize the fluid as in the case of the SFAI. With an additionalcircuit, the density of the liquid can be monitored. The phase-lockingis accomplished by using a saw-tooth wave signal to vary the frequencyof the excitation transducer around the desired resonance frequency of asingle resonance peak. The resonance peak is monitored as a function oftime and provides a-measure of the sound speed because the pipe diameteris known. In this case, it is not necessary to determine the frequencyspacing between any two consecutive peaks because the resonance spectrumis determined by the path length (pipe or tube diameter) and the soundspeed of the liquid. Therefore, the position of a single known peakdetermines the sound speed. The output of the frequency modulation is asignal that is amplitude modulated as it is swept through a resonancepeak. If the resonance is sharp then the amplitude modulation over theshort frequency sweep region is of high amplitude with a high medianamplitude value. For low amplitude or wide resonance peaks, the outputsignal is of lower median value with lower amplitude excursions.Therefore, by measuring the RMS value of the signal and AC coupling itso that the DC median value is filtered out, it is possible to derivethe resonance peak width. The median DC value provides a measure of theliquid density.

[0043]FIG. 3 is a composite resonance spectrum for a noninvasivemeasurement using the swept frequency apparatus and method of thepresent invention on a container having a finite wall thickness, andillustrates that liquid peaks can be studied independently of theresonances induced in the wall of the container if an appropriatefrequency region is selected. The following graph shows what a typicalspectrum looks like when a swept frequency measurement is made fromoutside a container with a finite wall thickness.

[0044]FIG. 4 is a graph of the physical properties of several liquidsmeasured in a static container. Decane and dodecane were investigatedsince both of these liquids are known to have similar properties tothose for oil. Acoustically, these liquids are far apart. The soundspeed and attenuation values are summarized in the TABLE. TABLE Soundspeed Density Attenuation Liquid m/s g/cm³ Np m⁻¹ S² × 10¹⁴ Decane 12630.73 5.7 Dodecane 1300 0.75 6.3 Water 1483 1.00 2.5 Water + 18% (NaCl)1550 1.01 6.0 Water + 26% (NaCl) 1585 1.02 30.0

[0045] The resolution for sound speed for the SFAI technique of thepresent invention is approximately ±2 m/s; this can be improved to 0.1m/s, if necessary. This difference between decane and dodecane permitsthem to be identified. Differentiating between water, brine and decane(or dodecane) is straight forward. The same data are presented in a3-dimensional graph in FIG. 3 for clarity.

[0046] Recent studies on the sound speed in pure hydrocarbons andmixtures using the traditional pulse-echo technique by Wang and Nur [5]show that sound speed in 13 n-alkanes, 10 1-alkenes, and 3 napthenehydrocarbon samples show that the sound speed decreases linearly withtemperature with slopes ranging from −3.43 to −4.85 [m/s]/° C. in atemperature range between −12° to 132° C. Therefore, if the temperatureis known, the sound speed can be corrected for temperature. In aseparate study [6] it is shown that the sound speed c for hydrocarbonscan be expressed as a function of temperature T and molecular weight Min atomic mass units as:$c = {c_{0} - {\left( {0.306 - \frac{7.6}{M}} \right)\quad T}}$

[0047] where, c₀ is a constant. This shows that it should be possible toidentify various hydrocarbons using sound speed if this quantity can bemeasured accurately.

[0048] In addition to sound speed, the SFAI technique can also determinesound absorption in the fluids, which provides an additional physicalparameter for oil characterization. Hydrocarbons also show pronouncedfrequency dependent sound absorption. The SFAI technique of the presentinvention is capable of this type of measurement as well.

[0049] A flow loop was employed to perform SFAI measurements underflowing conditions. A 4.5-in. diameter plastic tube was used in the flowloop. Water was used for the liquid because it is easier to work withthan crude oil. The measurement was also performed with vegetable oil.FIG. 5 shows the measurements under flowing conditions between 0 and 20gal./min. of water. The spacing between consecutive resonance peaks isseen to be the same for flowing and non-flowing water. This indicatesthat the sound speed does not change when the liquid is flowing. Thewidth of the resonance peaks are also observed to be the same,indicating that sound attenuation also remains invariable under flowingconditions. The difference between the two spectra is a slight shift ofthe entire pattern in frequency.

[0050] It is believed by the present inventor that the frequency shiftis due to a slight variation in the acoustical properties of the fluiddue to the flow boundary layer formed adjacent to the inner surface ofthe wall. This boundary layer tends to introduce a phase shift of thesound waves reflecting from the wall which can affect the standing-wavepattern formed inside the total fluid path length. The baseline drift tohigher amplitude toward the higher frequency side of the figure is aresult of the fact that the data presented are somewhat close to a wallresonance peak (see FIG. 3 hereof. It has been observed that theconstancy of the sound speed is observed from the FFT of the data.

[0051]FIG. 6 shows that SFAI measurements required for determining soundspeed can be made with fluids containing bubbles of gas. For thismeasurement, nitrogen gas was bubbled through the bottom of a Plexiglastube about 2-in. in diameter, and the measurements were made byattaching two transducers on the outside of the tube. To be noted isthat that the frequency spacing between consecutive resonance peaks doesnot significantly change, and that the spectra can be clearly observed(the measurements were made with little (˜1 ms) integration time);moreover, the periodicity can still be determined at relatively highbubbling rates. This indicates that the sound speed does not changeappreciably until the volume fraction of bubbles is large when thebubbling rate is too high. If the integration of the measurement isincreased by a factor of 10, the signal-to-noise ratio of the data wasfound to improve considerably, and the observed pattern for the bubblingliquid was found to be similar to the same liquid without introducedbubbles. This is because all the fluctuations due to the bubbles in themeasurements are averaged out, and up to a certain bubble rate, the SFAImeasurements are still quite reliable.

[0052]FIG. 7 is a plot of the shift in phase angle as a function of massflow, demonstrating that the apparatus of the present invention isuseful as a noninvasive flow meter; that is, by attaching transducers tothe outside of an existing pipe, the flow of the fluid therein can bemonitored.

[0053] For real-time (continuous) monitoring, it has been found to bemost useful to select a single resonance peak at an appropriatefrequency. FIG. 8 is a plot of resonance amplitude as a function offrequency for crude oil (upper trace) and for water (lower trace) in a2-inch diameter glass pipe. In the frequency range between 3.78 and 3.8MHz (enclosed by the rectangle), the particular resonator cavity (theinside of the pipe) reaches its maximum sensitivity in terms ofmonitoring changes in sound speed. There are many such frequenciesdispersed in a regular manner. A frequency shift of 5 kHz is observedbetween the data for crude oil and water. The SFAI technique of thepresent invention can easily resolve 1 Hz, therefore, allowing a soundspeed resolution of 1 part in 5000. Besides the shift in frequency, theresonance width also changes dramatically which indicates a largevariation in sound absorption. In addition, the minimum of the resonancealso changes due to a change in acoustic impedance mismatch and can berelated to liquid density. Electronic circuitry has been developed thatcan monitor all three parameters in a continuous manner. The: shift inthe baseline for the two plots (water and crude oil) is due to the factthat the acoustic impedance is different for the two fluids. The minimumvalue of the resonance provides a measure of the fluid density that canbe derived from the acoustic impedance mismatch between the pipe walland the fluid inside.

[0054] Thus, it is seen that frequency location of the resonance peaksvaries as a function of both the composition of the fluid and its flowrate. If a flow meter is desired, the composition must be determined tobe constant; this can be achieved by monitoring the peak spacing todetermine that the sound speed of the fluid remains relatively constantfor in-situ calibration. The calibration can also be performed using asmall section of the same pipe and a known liquid elsewhere in anyflowing system to derive the calibration information. In the flowcalibration, any resonance peak in a desired frequency range (preferablyin the frequency range in the middle of two wall resonance peaks) ismonitored as a function of the liquid flow. The wall resonance peakpositions are determined by the wall thickness. The present apparatuscan be calibrated for both high and low sensitivity measurements asfollows: For low frequencies (approximately 1 MHz), the shift of theresonance peaks is smaller than the shift observed at much higherfrequency (approximately 10 MHz). By observing multiple frequencyranges, it is possible to obtain different levels of sensitivity. Thiscalibration process is no different than for other transit-timeultrasonic flow meters where the fluid sound speed is to be determined.Once the apparatus is calibrated for flow, then both sound speed (andsound absorption) and fluid flow can be simultaneously monitored ifgreat accuracy in the measurement is not desired. For many practicalapplications, such as flow and composition monitoring in the oil(petroleum products) industry, an oil flow calibration provides adequateaccuracy. It is also possible, in principle, to extend the flowcalibration from one liquid, for example, water to oil. FIG. 8illustrates the difference in the resonance peaks for oil and water. Thewidths of the resonance peaks are different for the two liquids, andeach liquid can be identified based on the resonance characteristics ofjust a single resonance peak. Therefore, once the calibration for flowis completed for oil and separately for water, it is possible toextrapolate the flow rate when the flowing fluid is a combination of thetwo liquids because this quantity is intermediate between the twocalibrations. This is possible because the composition can be monitoredfrom a measurement of the peak spacing or by FFT of the resonance data,whereas the flow is measured by tracking the position of a singleresonance peak. These two measurements are independent of each other toa large extent in practice.

[0055] The present invention provides information at both low and highflow rates. Since the frequency shift of the peaks due to flow increaseswith frequency, for low flow rates it is convenient to use a higherfrequency range (≧5 MHz) where a small flow rate produces a measurableshift in peak frequency or phase shift of any selected resonance peak.By contrast, for higher flow rates, the resonance peak shift can belarge and one may lose track of the selected peak which is equivalent toexceeding a 360-degree phase shift. In this case, it is appropriate toobserve the data at a lower frequency the calibration information. Inthe flow calibration, any resonance peak in a desired frequency range(preferably in the frequency range in the middle of two wall resonancepeaks) is monitored as a function of the liquid flow. The wall resonancepeak positions are determined by the wall thickness. The presentapparatus can be calibrated for both high and low sensitivitymeasurements as follows: For low frequencies (approximately 1 MHz), theshift of the resonance peaks is smaller than the shift observed at muchhigher frequency (approximately 10 MHz). By observing multiple frequencyranges, it is possible to obtain different levels of sensitivity. Thiscalibration process is no different than for other transit-timeultrasonic flow meters where the fluid sound speed is to be determined.Once the apparatus is calibrated for flow, then both sound speed (andsound absorption) and fluid flow can be simultaneously monitored ifgreat accuracy in the measurement is not desired. For many practicalapplications, such as flow and composition monitoring in the oil(petroleum products) industry, an oil flow calibration provides adequateaccuracy. It is also possible, in principle, to extend the flowcalibration from one liquid, for example, water to oil. FIG. 8illustrates the difference in the resonance peaks for oil and water. Thewidths of the resonance peaks are different for the two liquids, andeach liquid can be identified based on the resonance characteristics ofjust a single resonance peak. Therefore, once the calibration for flowis completed for oil and separately for water, it is possible toextrapolate the flow rate when the flowing fluid is a combination of thetwo liquids because this quantity is intermediate between the twocalibrations. This is possible because the composition can be monitoredfrom a measurement of the peak spacing or by FFT of the resonance data,whereas the flow is measured by tracking the position of a singleresonance peak. These two measurements are independent of each other toa large extent in practice.

[0056] The present invention provides information at both low and highflow rates. Since the frequency shift of the peaks due to flow increaseswith frequency, for low flow rates it is convenient to use a higherfrequency range (≧5 MHz) where a small flow rate produces a measurableshift in peak frequency or phase shift of any selected resonance peak.By contrast, for higher flow rates, the resonance peak shift can belarge and one may lose track of the selected peak which is equivalent toexceeding a 360-degree phase shift. In this case, it is appropriate toobserve the data at a lower frequency region (≈1 MHz). The appropriatefrequency ranges depend on the particular pipe geometry and may bedetermined during the initial calibration process where a wide-bandfrequency scan is employed to determine the characteristics of the pipe(see FIG. 3 hereof). As mentioned hereinabove, it is preferable to usethe frequency regions between two wall resonance frequencies for bothflow and composition monitoring.

[0057] For a calibration of the system for flow, measurements (receiversignal amplitude and phase difference) are made with a flowing liquidfor several flow values and the entire frequency spectrum is monitored.Once this is done, the calibration information for the low and highfrequency ranges are extracted from these spectra and stored in themicrocontroller as terms of simple equations. From this any value canthen be interpolated for actual measurement.

[0058] Once the apparatus is calibrated for flow, both sound speed (andsound absorption) and fluid flow can be simultaneously monitored ifgreat accuracy in the measurement is not desired. For flow andcomposition monitoring of petroleum products, a simple flow calibrationwith oil can provide adequate monitoring. It is also possible, inprinciple, to extend the flow calibration from one liquid, for example,water to oil. FIG. 8 shows the difference in the resonance peaks for oiland water, and the liquid can be readily identified from the resonancecharacteristics of a single peak. Once the calibration for flow isperformed with oil and then with water, it is possible to correct theflow when the flowing fluid is a combination of any two because themeasured results will be between those for either liquid. This ispossible because the composition is monitored by measuring the peakspacing or FFT of the resonance data, whereas the flow is measured bytracking the position of a single resonance peak. These two measurementsare independent of each other.

[0059] The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching. The embodiments were chosen and described in order tobest explain the principles of the invention and its practicalapplication to thereby enable others skilled in the art to best utilizethe invention in various embodiments and with various modifications asare suited to the particular use contemplated. It is intended that thescope of the invention be defined by the claims appended hereto.

REFERENCES

[0060] 1. U.S. Pat. No. 5,767,407 for “Noninvasive Identification OfFluids By Swept-Frequency Acoustic Interferometry,” which issued toDipen N. Sinha on Jun. 16, 1998.

[0061] 2. F. Eggers and Th. Funck, “Ultrasonic relaxation spectroscopyin liquids”, Naturwissenschaften 63, 280 (1976).

[0062] 3. Dipen N. Sinha and Greg Kaduchak, “Noninvasive Determinationof Sound Speed and Attenuation in Liquids,” Experimental Methods in thePhysical Sciences, Volume 39, Academic Press (September 2001).

[0063] 4. U.S. Pat. No. 5,606,130 for “Method For Determining The OctaneRating Of Gasoline Samples By Observing Corresponding AcousticResonances Therein” which issued to Dipen N. Sinha and Brian W. Anthonyon Feb. 25, 1997

[0064] 5. U.S. Pat. No. 5,886,262 for “Apparatus And Method ForComparing Corresponding Acoustic Resonances in Liquids” which issued toDipen N. Sinha on Mar. 23, 1999.

[0065] 6. Zhijing Wang and Amos Nur, J. Acoust. Soc. Am. 89, 2725(1991).

[0066] 7. Z. Wang and A. Nur, Geophysics 55, 723 (1990).

What is claimed is:
 1. A method for monitoring the composition of afluid flowing through a vessel which comprises the steps of: (a)applying a continuous periodic acoustical signal to the outside of thevessel such that the acoustical signal is transferred to the flowingfluid, thereby generating vibrational resonance features having aplurality of maxima and minima therein; (b) detecting the vibrationalfeatures generated in the flowing liquid; (c) sweeping the continuousperiodic acoustical signal through a chosen frequency range whichincludes two chosen consecutive maxima among the vibrational resonancefeatures; and (d) measuring the frequency difference between the twochosen consecutive maxima of the flowing fluid.
 2. The method asdescribed in claim 1, further comprising the step of determining thefull-width-at-half-maximum of at least one of the two chosen consecutiveresonance features.
 3. The method as described in claim 1, furthercomprising the step of determining the acoustic impedance of the fluid.4. The method as described in claim 1, further comprising the step ofdetermining the ratio of the resonance feature minimum to the resonancefeature maximum.
 5. An apparatus for monitoring the composition of afluid flowing through a vessel which comprises in combination: (a) afirst transducer in acoustic contact with the outside surface of saidvessel for applying a continuous periodic acoustical signal to theoutside of said vessel such that the acoustical signal is transferred tosaid flowing fluid, thereby generating vibrational resonance featureshaving a plurality of maxima and minima therein; (b) a second transducerin acoustic contact with the outside of said vessel and located on theside thereof opposite to said first transducer for detecting thevibrational resonance features generated in the flowing liquid; (c) asweep generator for sweeping said first transducer through a chosenfrequency range which includes two chosen consecutive maxima among thevibrational resonance features; and (d) a data processor for determiningthe frequency difference between the two chosen consecutive maxima ofthe flowing fluid.
 6. The apparatus as described in claim 5, whereinsaid data processor determines the line width of at least one of the twochosen consecutive resonance features.
 7. The apparatus as described inclaim 5, wherein said data processor determines the acoustic impedanceof the fluid.
 8. The method as described in claim 5, wherein said dataprocessor determines the ratio of the resonance feature minimum to theresonance feature maximum.
 9. An apparatus for monitoring thecomposition of a fluid flowing through a vessel which comprises incombination: (a) a first transducer in acoustic contact with the outsidesurface of said pipe for applying a continuous periodic acousticalsignal to the outside of said vessel such that the acoustical signal istransferred to said flowing fluid, thereby generating vibrationalresonance features having a plurality of maxima and minima therein, andfor detecting the generated vibrational resonance features; (b) a sweepgenerator for sweeping said first transducer through a chosen frequencyrange which includes two chosen consecutive maxima in the vibrationalresonance features; and (c) a data processor for recording the frequencydifference between the two chosen consecutive maxima of the flowingfluid.
 10. The apparatus as described in claim 9, wherein said dataprocessor determines the line width of at least one of the two chosenconsecutive resonance features.
 11. The apparatus as described in claim9, wherein said data processor determines the acoustic impedance of thefluid.
 12. The apparatus as described in claim 9, wherein said dataprocessor determines the ratio of the resonance feature minimum to theresonance feature maximum.
 13. An apparatus for monitoring thecomposition of a fluid flowing through a vessel which comprises incombination: (a) a first transducer in acoustic contact with the outsidesurface of said vessel for applying a continuous periodic acousticalsignal to the outside of said vessel such that the acoustical signal istransferred to said flowing fluid, thereby generating vibrationalresonance features having a plurality of maxima and minima therein; (b)a second transducer in acoustic contact with the outside of said vesseland located on the same side thereof as said first transducer and in thevicinity thereof, for detecting the vibrational resonance featuresgenerated in the flowing liquid; (c) a sweep generator for sweeping saidfirst transducer through a chosen frequency range which includes twochosen consecutive maxima among the vibrational resonance features; and(d) a data processor for determining the frequency difference betweenthe two chosen consecutive maxima of the flowing fluid.
 14. Theapparatus as described in claim 13, wherein said data processordetermines the line width of at least one of the two chosen consecutiveresonance features.
 15. The apparatus as described in claim 13, whereinsaid data processor determines the acoustic impedance of the fluid. 16.The apparatus as described in claim 13, wherein said data processordetermines the ratio of the resonance feature minimum to the resonancefeature maximum.
 17. A method for monitoring the flow rate of a fluidthrough a vessel which comprises the steps of: (a) applying a continuousperiodic acoustical signal to the outside of the vessel such that theacoustical signal is transferred to the flowing fluid, therebygenerating vibrational resonance features having a plurality of maximaand minima therein; (b) detecting the vibrational resonance featuresgenerated in the flowing liquid; (c) sweeping the continuous periodicsignal through a chosen frequency range which includes two chosenconsecutive maxima in the standing- wave vibrational pattern; (d)recording the frequency difference between the two chosen consecutivemaxima to determine whether the composition of the fluid has changed;(e) correcting the location of the resonance peaks in response thereto;and (f) determining the frequency of one of the chosen resonance peaks,such that the flow rate of the fluid is determined.
 18. An apparatus formonitoring the flow rate of a fluid through a vessel which comprises incombination: (a) a first transducer in acoustic contact with the outsidesurface of said vessel for applying a continuous periodic acousticalsignal to the outside of said vessel such that the acoustical signal istransferred to said flowing fluid, thereby generating vibrationalresonance features having a plurality of maxima and minima therein; (b)a second transducer in acoustic contact with the outside of said vesseland located on the side thereof opposite to said first transducer fordetecting the vibrational resonance features generated in the flowingliquid; (c) a sweep generator for sweeping said first transducer througha chosen frequency range which includes two chosen consecutive maxima inthe standing-wave vibrational pattern; and (d) a data processor forrecording the frequency difference between the two chosen consecutivemaxima to determine whether the composition of the fluid has changed,for correcting the location of the resonance peaks in response thereto,and for determining the frequency of one of the chosen resonance peaks,such that the flow rate of the fluid is determined.
 19. An apparatus formonitoring the flow rate of a fluid flowing through a vessel whichcomprises in combination: (a) a first transducer in acoustic contactwith the outside surface of said pipe for applying a continuous periodicacoustical signal to the outside of said vessel such that the acousticalsignal is transferred to said flowing fluid, thereby generatingvibrational resonance features having a plurality of maxima and minimatherein, and for detecting the generated vibrational pattern; (b) asweep generator for sweeping said first transducer through a chosenfrequency range which includes two chosen consecutive maxima in thevibrational resonance features; and (c) a data processor for recordingthe frequency difference between the two chosen consecutive maxima ofthe flowing fluid to determine whether the composition of the fluid haschanged, for correcting the location of the resonance peaks in responsethereto, and for determining the frequency of a chosen resonance peak,such that the flow rate of the fluid is determined.
 20. An apparatus formonitoring the flow rate of a fluid through a vessel which comprises incombination: (a) a first transducer in acoustic contact with the outsidesurface of said vessel for applying a continuous periodic acousticalsignal to the outside of said vessel such that the acoustical signal istransferred to said flowing fluid, thereby generating vibrationalresonance features having a plurality of maxima and minima therein; (b)a second transducer in acoustic contact with the outside of said vesseland located on the same side thereof as said first transducer and in thevicinity thereof, for detecting the vibrational resonance featuresgenerated in the flowing liquid; (c) a sweep generator for sweeping saidfirst transducer through a chosen frequency range which includes twochosen consecutive maxima in the standing-wave vibrational pattern; and(d) a data processor for recording the frequency difference between thetwo chosen consecutive maxima to determine whether the composition ofthe fluid has changed, for correcting the location of the resonancepeaks in response thereto, and for determining the frequency of a chosenresonance peak, such that the flow rate of the fluid is determined. 21.A method for monitoring the composition of a fluid flowing at a flowrate through a vessel which comprises the steps of: (a) applying acontinuous periodic acoustical signal to the outside of the vessel suchthat the acoustical signal is transferred to the flowing fluid, therebygenerating vibrational resonance features having a plurality of maximaand minima therein; (b) detecting the vibrational features generated inthe flowing liquid; (c) sweeping the continuous periodic acousticalsignal through a chosen frequency range which includes one maximum amongthe vibrational resonance features; (d) measuring the frequency of themaximum of the flowing fluid; (e) measuring the flow rate of the fluid;and (f) correcting the frequency of the maximum for the rate of flow.22. An apparatus for monitoring the composition of a fluid flowing at aflow rate through a vessel which comprises in combination: (a) a firsttransducer in acoustic contact with the outside surface of said vesselfor applying a continuous periodic acoustical signal to the outside ofsaid vessel such that the acoustical signal is transferred to saidflowing fluid, thereby generating vibrational resonance features havinga plurality of maxima and minima therein; (b) a second transducer inacoustic contact with the outside of said vessel and located on the sidethereof opposite to said first transducer for detecting the vibrationalresonance features generated in the flowing liquid; (c) a sweepgenerator for sweeping said first transducer through a chosen frequencyrange which includes a chosen maximum among the vibrational resonancefeatures; (d) a flow meter for measuring the flow rate of the fluid; and(e) a data processor for determining the frequency of the chosen maximumand for correcting the frequency for the flow rate of the flowing fluid.23. An apparatus for monitoring the composition of a fluid flowing at aflow rate through a vessel which comprises in combination: (a) a firsttransducer in acoustic contact with the outside surface of said pipe forapplying a continuous periodic acoustical signal to the outside of saidvessel such that the acoustical signal is transferred to said flowingfluid, thereby generating vibrational resonance features having aplurality of maxima and minima therein, and for detecting the generatedvibrational resonance features; (b) a sweep generator for sweeping saidfirst transducer through a chosen frequency range which includes achosen maximum in the vibrational resonance features; (c) a flow meterfor measuring the flow rate of the fluid; and (d) a data processor fordetermining the frequency of the chosen maximum and for correcting thefrequency for the flow rate of the flowing fluid.
 24. An apparatus formonitoring the composition of a fluid flowing through a vessel whichcomprises in combination: (a) a first transducer in acoustic contactwith the outside surface of said vessel for applying a continuousperiodic acoustical signal to the outside of said vessel such that theacoustical signal is transferred to said flowing fluid, therebygenerating vibrational resonance features having a plurality of maximaand minima therein; (b) a second transducer in acoustic contact with theoutside of said vessel and located on the same side thereof as saidfirst transducer and in the vicinity thereof, for detecting thevibrational resonance features generated in the flowing liquid; (c) asweep generator for sweeping said first transducer through a chosenfrequency range which includes a chosen maximum among the vibrationalresonance features; (d) a flow meter for measuring the flow rate of thefluid; and (e) a data processor for determining the frequency of thechosen maximum and for correcting the frequency for the flow rate of theflowing fluid.
 25. A method for monitoring the flow rate of a fluidhaving a composition and flowing through a vessel which comprises thesteps of: (a) applying a continuous periodic acoustical signal to theoutside of the vessel such that the acoustical signal is transferred tothe flowing fluid, thereby generating vibrational resonance featureshaving a plurality of maxima and minima therein; (b) detecting thevibrational features generated in the flowing liquid; (c) sweeping thecontinuous periodic acoustical signal through a chosen frequency rangewhich includes one maximum among the vibrational resonance features; (d)measuring the frequency of the maximum of the flowing fluid; (e)determining the composition of the fluid; and (f) correcting thefrequency of the maximum for the composition of the fluid, whereby theflow rate of the fluid is determined.
 26. An apparatus for monitoringthe flow rate of a fluid having a composition and flowing at a flow ratethrough a vessel which comprises in combination: (a) a first transducerin acoustic contact with the outside surface of said vessel for applyinga continuous periodic acoustical signal to the outside of said vesselsuch that the acoustical signal is transferred to said flowing fluid,thereby generating vibrational resonance features having a plurality ofmaxima and minima therein; (b) a second transducer in acoustic contactwith the outside of said vessel and located on the side thereof oppositeto said first transducer for detecting the vibrational resonancefeatures generated in the flowing liquid; (c) a sweep generator forsweeping said first transducer through a chosen frequency range whichincludes a chosen maximum among the vibrational resonance features; (d)means for determining the composition of the fluid; and (e) a dataprocessor for determining the frequency of the chosen maximum and forcorrecting the frequency for the composition of the fluid, whereby theflow rate of the fluid is determined.
 27. An apparatus for monitoringthe flow rate of a fluid having a composition and flowing through avessel which comprises in combination: (a) a first transducer inacoustic contact with the outside surface of said pipe for applying acontinuous periodic acoustical signal to the outside of said vessel suchthat the acoustical signal is transferred to said flowing fluid, therebygenerating vibrational resonance features having a plurality of maximaand minima therein, and for detecting the generated vibrationalresonance features; (b) a sweep generator for sweeping said firsttransducer through a chosen frequency range which includes a chosenmaximum in the vibrational resonance features; (c) means for determiningthe composition of the fluid; and (d) a data processor for determiningthe frequency of the chosen maximum the flow rate of the fluid isdetermined.
 28. An apparatus for monitoring the flow rate of a fluidhaving a composition and flowing through a vessel which comprises incombination: (a) a first transducer in acoustic contact with the outsidesurface of said vessel for applying a continuous periodic acousticalsignal to the outside of said vessel such that the acoustical signal istransferred to said flowing fluid, thereby generating vibrationalresonance features having a plurality of maxima and minima therein; (b)a second transducer in acoustic contact with the outside of said vesseland located on the same side thereof as said first transducer and in thevicinity thereof, for detecting the vibrational resonance featuresgenerated in the flowing liquid; (c) a sweep generator for sweeping saidfirst transducer through a chosen frequency range which includes achosen maximum among the vibrational resonance features; (d) means fordetermining the composition of the fluid; and (e) a data processor fordetermining the frequency of the chosen maximum and for correcting thefrequency for the composition of the fluid, whereby the flow rate of thefluid is determined.
 29. A method for monitoring the flow rate of afluid having a composition and flowing through a vessel which comprisesthe steps of: (a) applying a continuous periodic acoustical signal tothe outside of the vessel such that the acoustical signal is transferredto the flowing fluid, thereby generating vibrational resonance features;(b) detecting the vibrational features generated in the flowing liquid;(c) sweeping the continuous periodic acoustical signal through a chosenfrequency range which includes a portion of one vibrational resonancefeature; (d) measuring the phase of the vibrational resonance featurerelative to that for the continuous periodic acoustical signalgenerating thereby a phase difference; (e) determining the compositionof the fluid; and (f) correcting the phase difference for thecomposition of the fluid, whereby the flow rate of the fluid isdetermined.
 30. An apparatus for monitoring the flow rate of a fluidhaving a composition and flowing through a vessel which comprises incombination: (a) a first transducer in acoustic contact with the outsidesurface of said pipe for applying a continuous periodic acousticalsignal to the outside of said vessel such that the acoustical signal istransferred to said flowing fluid, thereby generating vibrationalresonance features having a plurality of maxima and minima therein, andfor detecting the generated vibrational pattern; (b) a sweep generatorfor sweeping said first transducer through a chosen frequency rangewhich includes a portion of one vibrational resonance feature; (c) meansfor measuring the phase of the vibrational resonance feature relative tothat for the continuous periodic acoustical signal generating thereby aphase difference; (d) means for determining the composition of thefluid; and (e) a data processor for recording the phase difference andcorrecting the phase difference for the composition of the fluid,whereby the flow rate of the fluid is determined
 31. A method formonitoring the composition of a fluid flowing through a vessel at a flowrate which comprises the steps of: (a) applying a continuous periodicacoustical signal to the outside of the vessel such that the acousticalsignal is transferred to the flowing fluid, thereby generatingvibrational resonance features; (b) detecting the vibrational featuresgenerated in the flowing liquid; (c) sweeping the continuous periodicacoustical signal through a chosen frequency range which includes aportion of one vibrational resonance features; (d) measuring the phaseof the vibrational resonance feature relative to that for the continuousperiodic acoustical signal generating thereby a phase difference; (e)determining the flow rate of the fluid; and (f) correcting the phasedifference for the flow rate of the fluid, whereby changes in thecomposition of the fluid are identified.
 32. An apparatus for monitoringthe concentration of a fluid flowing through a vessel at a flow ratewhich comprises in combination: (a) a first transducer in acousticcontact with the outside surface of said pipe for applying a continuousperiodic acoustical signal to the outside of said vessel such that theacoustical signal is transferred to said flowing fluid, therebygenerating vibrational resonance features having a plurality of maximaand minima therein, and for detecting the generated vibrational pattern;(b) a sweep generator for sweeping said first transducer through achosen frequency range which includes a portion of one vibrationalresonance feature; (c) means for measuring the phase of the vibrationalresonance feature relative to that for the continuous periodicacoustical signal generating thereby a phase difference; (d) a flowmeter for determining the flow rate of the fluid; and (e) a dataprocessor for recording the phase difference and correcting the phasedifference for the composition of the fluid.